Learnings from a New Slim Hole LWD NMR Technology

Abstract This paper presents recent experience with a new 4 ¾-in logging-while-drilling (LWD) nuclear magnetic resonance (NMR) tool. Data from several wells drilled provided real-time operational insights and new petrophysical learnings. The new LWD technology presents a novel capability to measure NMR T1 and T2 distributions simultaneously with reduced sensitivity to drilling mud conductivity. Additionally, a real-time sensor motion-warning system is implemented to check if the drilling environment is suitable for NMR data acquisition. The real-time T1 and T2 spectra, communicated by mud pulse telemetry, compare favorably with the full dataset retrieved from the tool memory. For detailed back-to-back comparison, the LWD NMR measurements were followed by wireline (WL) NMR logging in three wells. In carbonate reservoirs, the main objective is the evaluation of the new tool's capability to resolve carbonate pore size with slow NMR relaxation rates. Another set of NMR logs included time-lapse repeat passes in a well drilled with oil-based mud (OBM) across a clastic reservoir. With the NMR property contrast between formation oil, water, and oil-based mud filtrate (OBMF), this sequence of measurements resolved rock types and provided unique insight to the process of mud filtrate invasion. In a carbonate reservoir, the LWD NMR data were processed to estimate micro-, meso-, and macropore volumes. The real-time partial porosity estimates are in excellent agreement with the core-calibrated wireline evaluation. Another carbonate formation contains high permeability layers embedded in a microporous rock. This layering is resolvable by NMR logs only. The real-time LWD NMR logs successfully located the high-quality zones as verified later by wireline logging. In the clastic reservoir, the LWD NMR data acquired while drilling indicated native light hydrocarbons, whereas the subsequent LWD reaming and wireline NMR passes showed a displacement of native hydrocarbons by OBMF. This fluid displacement appears as a shift of the free fluid signal from several seconds in the LWD log to about 600 ms in the WL NMR T2 spectrum. The native hydrocarbon signature observed by LWD NMR is in good agreement with the mud gas log. OBMF invasion occurs shortly after drilling as indicated by the differences observed in the LWD NMR relog data acquired several hours after the while-drilling pass. The wireline NMR data, logged about four days after drilling, shows advanced stages of OBMF invasion, including formation water displacement and wettability changes in intermediate and large pores. Finally, the environmental noise remains low in an LWD NMR dataset acquired in a well where the mud salinity changed by several folds, indicating that mud salinity has little effect on the quality of LWD NMR logs in slim holes. The new slim LWD NMR technology has demonstrated its robust capability to provide T1 and T2 logs by several examples. For the first time, a time-lapse comparison of NMR logs showed that OBM filtrate invasion could happen in silty sands with high capillary-bound fluid fractions.